Ask Dr. Universe – Eyeballs

Dr. Universe: How much does an eyeball weigh? – Rahman, 10, Tollygunge, India

Dear Rahman,

Our animal kingdom is full of different eyes. The human eye weighs less than an ounce. That’s about as heavy as 11 pennies. But I suppose the answer to your question really depends on which eyeballs you are curious about. Perhaps you are looking for an answer about the biggest animal eyes on our planet.

An elephant’s eye is about the size of a golf ball, but there are even bigger eyes. A gray whale’s eyes are about the size of a baseball. But they still aren’t the biggest eyes. Those belong to the giant squid.

I decided to ask my friend Kirt Onthank exactly how much giant squid eyes weigh. He studied cephalopods, which include squid, as a student at Washington State University and now teaches biology at Walla Walla University.

“I don’t know the exact answer,” Onthank said. “But we can get a really good estimation.”

He said the largest giant squid was actually measured from a photograph. No one actually weighed it. But we do know its eye had a diameter of 10.5 inches, which is just a little bigger than a basketball.

While human eyes are made up of a more jelly-like material, a squid’s eyes are pretty much all seawater. Knowing this, we can estimate its weight.

After a little math, it comes out to about 22.7 pounds—more than 3,000 pennies.

“That is one really big eye,” Onthank said.

Colossal squid have even bigger eyes that weigh in at about 25.3 pounds. Even though their eyes are much bigger than yours, they still have some of the same parts.

Both squid and people have a lenses, irises, pupils to let light in, and retinas to capture the light and help send a message to your brain. One thing squids don’t have is eyelids.

Exactly why colossal squid need the world’s biggest eyeballs is a question some scientists are still investigating. The best theory is that their eyes are geared to see sperm whales, Onthank said.

Sperm whales have eyes that are only 7 centimeters in diameter. They are super tiny compared to their 40-foot-long bodies. They depend on sound waves that bounce off things to figure out where they are and to find food. And their favorite food is squid.

A squid’s big eye helps it detect very dim light deep in the ocean. It turns out, there are also some bioluminescent creatures in the deep waters. Bioluminescent creatures make their own light through a chemical reaction that allows them to glow in total darkness. If there was a sperm whale around, it would disturb some of these creatures who give off light.

The squid can use its big eyes to take in the light and receive a signal that might just help it avoid becoming a sperm whale’s lunch.